While the background of the present invention is discussed in terms of plasma display panel (PDP) applications, it is understood that the present invention is useful in flat panel display applications, in general.
The PDP typically comprises a pair of forward and backward insulation substrates arranged in opposition to each other to form a plurality of cells as display elements each defined by the insulation substrates supported with a constant interval and cell barriers arranged between the insulation substrates, two crossing electrodes disposed on internal surfaces of the insulation substrates with a dielectric layer interposed between the electrodes which cause electric discharge in a plurality of cells by application of an alternating current. Due to this application of alternating current, phosphor screens formed on the wall surface of the cell barrier emit light and display images which are passed through the transparent insulation substrate (typically called the front glass substrate or plate).
One area of concern for PDP manufacturers is display contrast, which affects the ultimate picture viewed by the consumer. To improve the display contrast, it is essential to decrease the reflection of external light from the electrodes and conductors arranged on the front glass substrate of the PDP device. This reflection decrease can be accomplished by making the electrodes and conductors black as viewed through the front plate of the display.
Furthermore, another area of concern for PDP manufacturers is environmental in nature and is the lead and cadmium contained in the prior art black conductor compositions and black electrodes of the PDP device. It is desirable to reduce and/or eliminate the lead and cadmium contained in the black conductor compositions and electrodes while still maintaining the required physical and electrical properties of the compositions and electrodes.
For example, in Japanese Kokai Patent No. HEI 10[1998]-73233 and its division Japanese Kokai Patent No. 2004-158456, light-forming black electrode compositions containing conductive particles consisting of at least one substance chosen from ruthenium oxide, ruthenium polyoxide, or their mixture and an inorganic binder, black electrodes using such compositions, plasma display panels using such black electrodes, and a method for making such a plasma display panel are disclosed. These literature references are not directed to lead-free black conductive compositions. In these references, there are no descriptions on lead-free black conductive compositions in terms of properties such as the appearance and strength of black electrodes obtained by sintering the compositions, electrical properties such as resistance, and a balance of all the properties for PDP electrodes.
Japanese Patent No. 3510761 discloses alkali-developable photocurable conductive paste compositions for plasma display panels, easily forming high-precision electrode circuits on large areas by photolithography and firing below 600° C. Such compositions contain (A) copolymer resins obtained by the addition of glycidyl acrylate and/or glycidyl methacrylate to copolymers of methyl methacrylate and methacrylic acid and/or acrylic acid; (B) photochemical polymerization initiator; (C) photopolymerizable monomer; (D) at least one conductive metal powder selected from Au, Ag, Ni, and Al; (E) glass frit; and (F) a phosphoric acid compound. Particularly in this literature, a low-melting glass frit is described using lead oxide as the preferred main component, while there are no descriptions of lead-free conductive compositions, especially black conductive compositions.
Japanese Patent No. 3541125 discloses alkali-developable curable conductive paste compositions that have excellent adhesion to the substrate after being fired, with adhesion between layers, suppression of curling, easy formation of high-precision conductive circuit patterns in large areas by photolithography, and are especially useful for forming underlayer electrode circuits of bus electrodes formed on the front substrate of plasma display panel. These compositions consist of: (A) carboxy-group-containing resins; (B) photopolymerizable monomer; (C) photochemical polymerization initiator; (D) silanol-group-containing synthetic amorphous silica fine powder; (E) conductive powder; and if needed (F) heat-resistant black pigment; (G) glass frit; and (H) stabilizer. In particular, this literature has a description of a low-melting glass frit using lead oxide as the preferred main component, while there are no descriptions of lead-free conductive compositions, especially black conductive compositions.
Japanese Patent No. 3479463 discloses photocurable conductive compositions providing excellent adhesion on a substrate in steps involving drying, exposure, development and firing, and resolution, satisfying the need for both a sufficient conductivity and blackness after being fired and discloses plasma display panels (PDP) with formation of the underlayer (black layer) electrode circuit using such compositions. The basic first embodiment of the compositions described in this literature contains (A) black conductive microparticles having a surface area to weight ratio larger than 20 m2/g and containing at least one substance chosen from ruthenium oxide or other ruthenium compound, copper-chromium black composite oxide and copper-iron black composite oxide, (B) an organic binder, (C) a photopolymerizable monomer, and (D) a photochemical polymerization initiator. The second embodiment contains (E) inorganic fine particles in addition to the above components. In this literature, with respect to this composition, the inorganic fine particles (E) contain, as needed, glass powder with a softening point of 400-600° C., conductive powder, heat-resistant black pigment, silica powder, etc. However, in the compositions of this literature, glass powder is not an essential component, and even when a glass component is added, lead oxide is described as the preferred main component, with no disclosure of lead-free black conductive compositions.
Japanese Patent No. 3538387 discloses photocurable conductive compositions having excellent storage stability, providing excellent adhesion on substrates in the different steps of drying, exposure, development and firing, and resolution, and satisfying the need for both sufficient blackness after being fired, and discloses plasma display panels (PDP) with the formation of the underlayer (black layer) electrode circuit using such compositions. The basic first embodiment of these photocurable resin compositions contains (A) tricobalt tetroxide (Co3O4) black microparticles, (B) organic binder, (C) photopolymerizable monomer, and (D) photochemical polymerization initiator. The second embodiment contains (E) inorganic microparticles in addition to the above components. In this literature, with respect to this composition, the inorganic fine particles (E) contain, as needed, a glass powder with a softening point of 400-600° C., conductive powder, heat-resistant black pigment, silica powder, etc. However, the compositions of this literature do not contain conductive materials such as ruthenium oxide, and glass powder is not an essential component. Even when a glass component is added, lead oxide is described as the preferred main component, with no disclosure of lead-free black conductive compositions.
Japanese Patent No. 3538408 discloses photocurable conductive compositions having excellent storage stability, providing excellent adhesion on substrates in different steps of drying, exposure, development and firing, and resolution, and satisfying the need for both sufficient conductivity and blackness after being fired, and discloses plasma display panels (PDP) with the formation of the underlayer (black layer) electrode circuit using such compositions. The basic first embodiment of these photocurable resin compositions contains (A) black inorganic microparticles such as inorganic binder-coated ruthenium oxide or another ruthenium compound, copper-chromium black composite oxide, copper-iron black composite oxide, cobalt oxide, etc., (B) organic binder, (C) photopolymerizable monomer, and (D) photochemical polymerization initiator. The photocurable compositions described in this literature are characterized by containing inorganic binder-coated black inorganic microparticles (A). The inorganic binder-coated black inorganic microparticles (A) are obtained by pulverizing molten materials of inorganic microparticles and an inorganic binder, with an inorganic binder having a softening point of 400-600° C. and glass powder with lead oxide as the main component being described as preferred, but with no disclosure of lead-free black conductive compositions.
Japanese Kohyo Patent Application No. 2003-521092 (Thompson plasma) disclosed a method for forming the face plate of a plasma display panel having a step in which a paste containing a metal powder and a mineral-based coupling agent is used to deposit an electrode, as well as a step in which the deposited electrode is sintered. This invention is characterized by the fact that the composition of the mineral-based coupling agent and sintering conditions are adjusted such that the coupling agent is recrystallized after the deposited electrode is sintered. The subject of this reference is not the lead-free coupling agent and is characterized by the fact that the temperature for sintering the electrode does not exceed 470° C. Also, the purpose of this reference is to eliminate the yellow color forming during the heat treatment of the substrate.
Japanese Kokai Patent Application No. 2002-367518 disclosed a type of electrode material used for an Ag electrode or black stripe, which can prevent blisters during sintering by using a crystallized glass frit that acts as a coupling agent. This invention is characterized by using a fluorine-containing glass frit, but the subject of this invention is not a lead-free glass frit. Therefore, there is also no description regarding the manufacturing method of a lead-free black electrode or the lead-free black electrode itself. In particular, the manufacturing method of a black electrode containing a lead-free crystalline glass binder is not disclosed in these references.
Japanese Kokai Patent Application No. 2002-373592 disclosed a type of electrode material used for an Ag electrode or black stripe, which can prevent blisters during sintering by using a crystallized glass frit that acts as a coupling agent. This invention is characterized by using a lead-containing glass frit, but the subject of this invention is not the lead-free glass frit. Although a lead-free glass frit is mentioned in this reference, there is no description regarding a lead-free black electroconductive composition using a lead-free bismuth-based crystallized glass frit or lead-free black electrode using the aforementioned lead-free black electroconductive composition.
Japanese Kokai Patent Application No. 2003-223851 (patent reference 11) disclosed a type of plasma display substrate structural body and its manufacturing method. For this substrate structural body of a plasma display with an electrode and dielectric layer formed on a substrate, the dielectric layer is formed by a glass layer with a low melting point, the electrode is formed by a metal layer containing crystallized glass, and the electrode and dielectric layer are sintered at the same time. According to this reference, snaking, disconnection, and floating of the bus electrode or black electrode can be prevented by reducing the number of rounds of sintering and by using crystallized glass. However, there is no detailed description regarding the composition, content, etc. of the crystallized glass, and the subject is not a lead-free glass composition. In addition, although it is disclosed that the crystallization peak temperature of the crystallized glass is lower than 560-590° C., the specific temperature is not disclosed at all.
The present inventors therefore desired to provide novel black conductive compositions to be used in flat panel display devices, for forming black electrodes having a desirable balance of all the preferred electrode properties including electrode pattern properties, blackness, resistance, etc. Furthermore, the present inventors desired to provide such compositions and electrodes formed therefrom which are lead-free. Still further, the present inventors desired to provide flat panel display devices comprising such electrodes.